1. Field of the Invention
An aspect of the present invention relates to a high temperature fuel cell system having a cooling apparatus, and a method of operating the same, and, more particularly, to a high temperature fuel cell system that increases a moisture removal efficiency of the fuel cell system by reducing the temperature of a hydrogen containing gas entering the fuel cell system using a moisture removing apparatus installed at a bypass line of the hydrogen containing gas, and a method of operating the same.
2. Description of the Related Art
A fuel cell is an electrical generation system that transforms chemical energy directly into electrical energy through a chemical reaction between oxygen and hydrogen contained in a hydrocarbon group material, such as methanol, ethanol, or natural gas. Types of fuel cells include phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and polymer electrolyte membrane or alkaline fuel cells, each of which is classified according to the type of electrolyte used therein. These fuel cells all operate on similar basic principles, but have their own operating characteristics according to their types of fuel, operating temperatures, catalysts, and electrolytes.
The polymer electrolyte membrane fuel cell (PEMFC) has the advantages of superior output, a low operating temperature, rapid starting, and speedy response time compared to other fuel cells, and is, therefore, a common fuel cell in the automotive and portable device industries and in residential and small commercial applications.
The polymer electrolyte membranes of the PEMFC have generally been formed of a polymer electrolyte such as perfluorosulfonic acid polymer, such as NAFION™. The polymer electrolyte membrane functions as a high ionic conductor by containing an adequate amount of water.
Conventional PEMFCs are generally operated at a temperature below 100° C., e.g., about 80° C., due to a drying problem of the polymer electrolyte membrane. Conventional PEMFCs have the following problems due to the low operating temperature. A hydrogen-rich gas, which is a representative fuel for the PEMFC, is obtained by reforming an organic fuel such as natural gas or methanol. In the reforming process, carbon dioxide and carbon monoxide may be present as byproducts in the hydrogen-rich gas, and carbon monoxide may poison catalysts contained in a cathode and an anode. The electrochemical activity of the poisoned catalysts is greatly reduced, and, accordingly, the operational efficiency and lifetime of the PEMFC are seriously reduced. The lower the operating temperature of the PEMFC, the worse the catalyst poisoning by carbon monoxide tends to be.
When the operating temperature of the PEMFC is higher than 150° C., catalyst poisoning by carbon monoxide may be avoided, and the water management of the PEMFC may be more easily controlled. Therefore, the fuel reformer may be miniaturized and the cooling apparatus can be simplified, thereby miniaturizing the overall PEMFC system.
The electrolyte membrane used for a high temperature fuel cell uses acid instead of water as a hydrogen ion conductor. Accordingly, the maintenance of the acid content in the electrolyte membrane is very important. Since acid is water soluble, it is easily leached through an electrode on contact with water. Accordingly, direct contact between the acid and the water should be avoided. Under typical operating conditions, however, water may be present as a vapor since the operating temperature of the fuel cell is approximately 150° C.
In this case, when the operation of the fuel cell is stopped, water is generated at the electrode as the fuel cell stack cools. Therefore, the acid contained in the electrolyte membrane will be leached away. Accordingly, the prevention of water generation at the electrode layer even if the fuel cell is cooled is important.
U.S. Pat. No. 6,492,044 has disclosed a method of including a desiccant in the fuel cell to prevent the generation of water at the electrode when the fuel cell cools down. However, this method may not sufficiently remove water vapor from pores of the electrode. In addition, more desiccant must be used as the number of unit fuel cells increases.